Or is this just another example of rule of cool taking precedence over practicality? What structural or design flaws might exist in a hot knife, and what problems might they pose for a soldier trying to use one?

These knives would be used to perform battlefield amputations as well as to cauterize wounds to prevent blood loss and infection in a near-future setting where biological warfare has become more common.

$\begingroup$Also, what timeframe/tech/magic? Modern "combat" knives are more of a tools than weapons, their actual combat use being relegated to emergencies. Depending on temperature and practicality over ordinary tools, there might be uses for such tool ranging from cooking to welding.$\endgroup$
– M i echNov 28 '16 at 21:14

1

$\begingroup$I commented on this under Tezra's answer. These knives would be used to perform battlefield amputations as well as to cauterize wounds to prevent blood loss and infection in a near-future setting where biological warfare has become more common.$\endgroup$
– Z.SchroederNov 28 '16 at 21:22

7 Answers
7

This might be further into the future than your planned setting, but one important reason for a heat knife would be countering enemy cybernetics and nanotech.

Computers and electronics can prove surprisingly tough to disable with direct physical force, especially given a damage path the size of a knife's stab. Small targets like cranial backup chips or health monitoring implants (which would be vital targets in covert operations) can be missed entirely, and you may not be accurate or lucky enough to deal critical damage to larger targets like cyberlimbs. However, the heat tolerance of circuitry is fairly low and heat propagates naturally, meaning that a heat knife would have a good chance to damage, disable or destroy electronics that the simple strike would have missed.

This applies doubly for nanotech; because of nanobots' tiny size, external heat rapidly cooks them and renders them useless. Medical nanotech can put a soldier back on their feet in minutes, but the heat knife's combination of mass tissue damage and local medichine destruction would keep an enemy incapacitated for longer. A heat knife could also serve as a last defence against certain kinds of nanoswarms.

The implications on the knife's durability are potentially troublesome, but the advantages of the heat knife in a cybernetic battlefield are potentially useful enough to compensate.

Well, aside from constantly heating/cooling the knife making it more brittle, and most knives will lose their solidity if heated to anything usefully high (though tungsten should work for this)... and the massive amount of energy used to heat up these weapons... what tactical advantage are you trying to gain? Set people of fire while you stab them? (a sparker wold be better for that) To make it act like an impromptu welder? (blast clay will probably be easier to carry/use, or some thermite)

You would need a very good insulator hilt (or low heat knife) to not burn the hand of the wielder. And if not made of a very high quality material, it will likely break when they need it most. You're main problem is that it will either be too expensive to make, or too low quality to be useful past single use.

EDIT:

So yes, this would cauterize wounds, but in medical use, you wouldn't use a knife (as precision cuts need to be open long enough to remove shrapnel) you would want some form of bone saw (something to try and cut through limb and bone quickly and cleanly). If this is to be performed on yourself, a tourniquet might be a better choice until you can get real medical help.

$\begingroup$Well, I'm imagining their primary function, since actually engaging in melee combat is becoming less and less common as warfare evolves alongside technology, is not to cut people or metal more efficiently, but to provide cleaner, less jagged battlefield amputations that will prevent infections without using antibiotics and halt the loss of blood. These knives are being used on a hypothetical future battlefield where chemical and biological warfare are more common, and burning dead bodies with flamethrowers or vehicle-mounted chemical lasers is common practice in war.$\endgroup$
– Z.SchroederNov 28 '16 at 21:18

$\begingroup$@Z.Schroeder Can you add that to the question body? Since it will affect the quality of answers.$\endgroup$
– TezraNov 28 '16 at 21:24

The first thing that came to my mind is that a super-heated knife is scary. Imagine you're confronting someone in hand to hand combat and they pull out a glowing hot knife. If that didn't freak you out enough to leave them alone, it might at least make you uneasy and would probably make your fighting sloppy. You can debate the actual effectivness of such a weapon, but I think that the psychological side would be the knife's primary strong suit.

As a side note

This idea even has a similar real life example in the taser which often have warning arcs which consist of a large spark to show the attacker that the weapon is charged and will be used if needed.

WARNING ARC
The warning arc increases voluntary surrenders and helps stop conflicts from escalating. It issues an audible warning directly over the front of live cartridges.
“All of our initial field uses of the X2 resulted in voluntary surrenders upon display of the warning arc… This was dramatically different than our previous experience with the X26.”
— Kevin Sailor, Westminster Police Department (taken from here)

To be clear, superheated is a technical term which means a fluid at boiling temperature at such pressure that it remains a liquid. I will disregard this here.

Heat is energy. Adding energy to metal causes it to do two things:

-Changing the intensity and spectrum of em-radiation.

In simple terms, hot metal starts to glow well before it becomes fluid. It starts with a faint reddish glow at $500°C$, turning to bright white at around $1300°C$

-Changes the vibration intensity of the atoms.

Atoms, like more complex molecules, vibrate, and the harder they vibrate, the easier the material is to manipulate. Steel at $700°C$ and above will be much more easy to bend than cold steel. At $1000°C$ (this is roughly the temperature jet fuel burns at) it might even bend if you swing it around too hard. This means that the blade cannot be heated at such temperatures that it glows visibly, and still retain usefulness as a combat weapon.

At a $100°C$ it is unlikely to bend easily, but at this point it will just cauterise any wounds it causes, and is unlikely to do any kind of damage that a cold blade will not. It will burn the skin, which can cause infections in the long run, but the long run is not what a knife is for.

It unlikely to provide a useful amputation tool, as blistering caused by burns in a combat situation can be dangerous, even deadly.
A tourniquet and sterilized blade are a better idea.

$\begingroup$I've heated cast iron metal to over 200C (to 450F, 230C) and have never noticed any pale glow at all. Not even the bottom of the oven which is presumably even hotter as it's heated by the burner.$\endgroup$
– JohnnyNov 28 '16 at 23:58

$\begingroup$@Johnny 200C is the point where it starts to emit light in the visible spectrum. Like I said, it's at higher temperatures that the glow becomes obvious.$\endgroup$
– FeyreNov 29 '16 at 9:39

$\begingroup$do you have a reference for that? In my experience, metal has no visible glow at all at 200C (even in the dark), and the references I can find for hot steel/iron say that a visible glow doesn't happen until around 400C - 500C$\endgroup$
– JohnnyNov 29 '16 at 17:11

$\begingroup$@Johnny Turns out I checked the wrong chart, some charts add tempered steel colours (which isn't from radiation). So you were right.$\endgroup$
– FeyreNov 29 '16 at 17:20

-Easier to injure someone, disable them, or render them temporarily harmless with attacks that wouldn't achieve much with a regular blade. You might not be able to land a killing blow, but burning their skin could be mighty annoying and give you an edge.

CONS

-Dramatic risk of hurting the user

-The heating element in the sheath/handle would be very uncomfortable (best case) and hazardous to just carry around. If the energy and technology required are handwaved (as well as the strength of the knife blade), you're still carrying a nearly forge-hot toaster on your waist. That would make me nervous.

A burn is more prone to infection and more difficult to heal, thus needing more medicine and doctor care than a cut.
Since the setting with common biological warfare, it could be useful to decrease enemy's disease resistance and overwork theirs doctors.

However, heating the blade, even if it's just during fight, will weaken it, making it more prone to break or blunder.

it is not meant as a weapon. the heating is for cutting some kind of packaging that needs to be heat sealed as you cut, perhaps the new super armor has a liquid core that leaks after it takes a hit but can be heat sealed shut. use as as weapon would be an afterthought. or maybe it is for removing armor from the injured and you need a hot knife to cut the new armor, kevlar can be cut with scissors maybe the new stuff is not so easy.

pure psychological, it makes for a crappy weapon but it is scary as all hell, designs like this have happened before. it would be abandoned fairly quickly but some diehards might still have one.